Automatic tuning control



lJuly 12, 1938.

M. G. 'CROSBY 2,123,716

AUTOMATIC TUNING CONTROL 3 Sheets-Sheet l Filed April 1e, 1935 INVENTOR.MURRAY G. CROSBY BY ,2f-@MW ATTORNEY.

July l2, 1938. M. G. CROSBY AUTOMATIC TUNING CONTROL Filed April 16,1935 3 Sheets-Sheet 2 NVENTOR MURRAY @CROSBY Y v v ATTRNEY.

July l2, 1938. M. G. CROSBY AUTOMATIC TUNING CONTROL Filed April 1e,1955 s sheets-sheet s INVENTOR.

MURRAY GJCROSBY F/Rsr Y DE TE c TOR ATTORNEY.

Patented July 12, 1938 UNITED STATES ATINT OFFICE AUTOMATIC TUNINGCONTROL Delaware Application April 16, 1935, Serial No. 16,591

17 Claims.

This invention relates to a novel method of and means for improving theoperation of an automatic tuning control device of the type disclosed inmy U. S. application Ser. No. 616,803,

5 filed June 13, 1932 and issued on Dec. 29, 1936 as U. S. P. 2,065,565.In the present method and means, the incoming signal energy is utilizedto turn the automatic tuning control on at the signal maximums and toturn the automatic tuning control ofiC during the signal minimums. Thetuning means, the control means for the tuning means, and the associatedcircuits are applicable to receivers of frequency or phase or amplitudemodulated oscillations. The method and means of the present inventionmay be readily applied to new receivers or to receivers known heretoforein the art.

In the prior art, automatic tuning control methods and means and/orautomatic frequency control methods and means have been subject to thedisadvantage that, during the intervals of low signal strength caused byfading or other phenomena, noise or other interfering signals may becomepredominant over the signal energy in the receiver, and thereby causethe tuning control and/or frequency control to be shifted outside of itsproper range. Hence, when the signal energy reappears, the receiver maybe out of tune, and in some cases so far out of tune that the controlcircuits may be unable to start functioning to retune the receiver.

It is the purpose of the present invention to provide a method and meanswhereby the reduction of the signal energy below a certain level 35causes the tuning control or frequency control system to be switched offso that the tuning remains fixed at the point at which it was left whenthe signal disappeared until the signal energy reappears. In thismanner, the frequency control means or tuning control means functions onthe signal peaks and is fixed, that is, ceases to operate during thetime minimum signal strength is received. Consequently, the effects dueto fading are removed- The novel features of my invention have beenpointed out in the claims appended hereto. The nature of my inventionand the operation thereof will be understood from the followingVdetailed description thereof and therefrom, when read in connectionwith the drawings throughout which like reference characters indicatelike parts insofar as possible and in which:

Fig. l illustrates a receiver of the heterodyne type including a novelfrequency control circuit,

(Cl. Z50-20) a novel control system for the frequency control circuit,and signal indicating means;

Fig. 2 shows a modification of the arrangement of Fig. l;

Figs. 3 and 4 illustrate modifications of portions of the circuit inFig. 2; while Figs. 5, 6 and 7 are vector diagrams illustrating theoperation of certain portions of the receiver circuits.

The circuit of Fig. l shows a specific embodiment of my invention. Thereceiver of Fig. 1 may comprise an aerial system A connected as shownwith a radio frequency amplifier I. The radio frequency amplifier I maybe connected as shown to a first detector 2. The first detector 2 iscoupled at its output to an intermediate frequency amplifier and filter3. The intermediate frequency energy is supplied to a carrier filtercircuit 4. The circuit Il may also include an amplitude limiter of anyknown type. The intermediate frequency energy in 4 is stripped of allside frequencies by the filter therein and is fed in phase opposition byway of a transformer T to the control grids of a pair of thermionicdifferential detectors 'I and 8. The intermediate frequency energy fromthe output of the intermediate frequency amplifier 3 is fed directly byway of a transformer T2 cophasally to the control grids of differentialdetectors l and 8. The differential control voltage or resulting Voltagefrom the anodes of the detectors l and 8 are fed to resistances Il andI8 and from said resistances by Way of contacts I I and I'Z and afrequency control line L, and time constant circuit I3, comprisingresistances R and a capacity C, to the control grid of a modulator tube6. The anode ofthe tube 6 is connected with inductance I 6, which may becoupled to the tuning inductances of the local oscillator 5 whichsupplies local oscillations to the rst detector for demodulationpurposes.

The manner in which the differential currents act in differentialdetectors I and 3 to produce a resultant in I'I and I8 which operatesthrough modulator 6 to control the local oscillator will be understoodby reference to the vector diagrams in Figs. 5, 6 and 7. The filteredand limited energy is applied from 4 in phase opposition to controlgrids of tubes 'I and 8, as shown by Vectors EI, E2 of Fig. 5. At thesame time, unfiltered carrier energy is supplied from the output of theintermediate frequency amplifier 3 to transformer T2 and from thesecondary thereof in phase to the control grids of tubes I and 8`, asindicated by the vector ES of Fig. 5. The resultant voltages may ,now berepresented by the vectors E3 and E4 when the intermediate frequencyobtained by beating oscillations from 5 with the received wave is at thepeak of the filter in 4. Now, if the frequency of the oscillator 3 orreceived signal should change, the vectors of Fig. 5 may shift topositions EI and E2, as shown in Fig. 6, due to the phase change of EIand E2, as effected by the crystal filter. The resultants E'3 and E4 areno longer equal. Differential currents appear in I1 and I8 and actthrough the line L, time control device I3 and modulator 6 to retune thelocal oscillator 5. If the shift of frequency occurs in the oppositedirection, the vectors might be as shown in Fig. 7 and the differentialcurrent would act in the opposite-direction to retune the oscillator 5to compensate `for said shift. The time constant of the line L and timeconstant device I3 is such that ,it responds to frequency changes butdoes not respond to frequency modulations. The plate impedance of tubeIiY is placed across coil I6, which is coupled to the oscillatingcircuit of oscillator tube 5. Thus, the control voltage producedin 6functions to change the effective inductance of the Vtuned circuit of 5and thereby vary the frequency of the high frequency oscillator to apoint such that the differential detectors 1, 8 cease to produce controlvoltage.

Thus far, the circuit including the tuning control means, the filtercircuit, the4 limiter, etc., are substantially thesame as thecorresponding elements of the automatic frequency control circuitdisclosedin my aforesaid patent. In the present circuit, however, arectifier tube 9 has its control grid connected as shovvn to the outputof the intermediate frequency amplifier and filter 3 to be energized byunfiltered signal voltage, so that relay I0 may be set to operate at anypredetermined'value of signal voltage. Hence, the rise in signalAvoltage will cause the relay to close the contacts II and I2 byattracting the same toward the armature of relay I0. This will connectthe output of the differential detectors 1A .and 8 to the line L andthereby apply the differential voltage to the input of the modulatortube 6. Now, assume that the signal voltage falls ofi or decreases; therelay contacts II and I2 under action of vsprings S, open, breaking thecircuit between L and I1, I8, so that condenser C of time constantcircuit I3 remains charged and holds the bias of tube Gat a fixed pointuntil the signal strengthrises again. The function of the relay 9, I0,then, is to disconnect condenser C from the-differential detector platesso that C will not discharge through I1, I8 during fading intervals. Ifthe relay 9, I were not present, C would quickly discharge throughresistors I1 and I8. 'I'his would change the tune of 5, that is, detunethe receiver. When the signal returned, C would have to be rechargedbefore the receiver would be automatically tuned to the signal. Thus,the tuning control might actually be tuned outside of the range Withinwhich it Would automatically operate.

The tube I4 may be an audio detector and serves to produce indicationsof the received Wave for recording purposes. 'I'he detector I4 may haveits grids adjusted on the linear portions of their dynamiccharacteristics. The unfiltered signal energy is fed from the output of3 to one grid of tube I4. The filtered or carrier stripped energy is fedfrom the output of S4 to another Vgrid of the tube I4. The filtered orstripped energy may be passed through the phase shifter I9 to obtain the4desired phase relation betweenthe Vaddition to my tuning controldevice.

filtered and unfiltered energies on the two grids. Thisphase relationwill be 90 or any odd multiple of 90, such as 270 or 450 etc., for thereception of phase modulation and 0 or any multiple of 180, such as 360or 540 etc., for amplitude modulation. The demodulator I4 may have itsoutput coupled as shown to a low frequency amplifier I5 which in turnmay be coupled to an indicator. The demodulator I4 and its associatedcircuit has been completely described and claimed in my U. S.application Ser. No. 716,469, filed March 20, 1934, Patent No.2,063,588, December 8, 1936, and needs no further description in thispaper.

The circuit of Fig. 2 shows the application of my novel methodand meansto a receiver which includes an automatic volume control device in Theelements of the circuit of Fig. 2 are designated by numbers orcharacters utilized to designate the corresponding elements of Fig. 1.The circuit of Fig. 2 corresponds to the circuit of Fig. 1, except forVthe differences which Will now be described. 'The detectors 1 and 8 ofFig. 1 are in Fig. 2 replaced byV multi-grid detectors connected asshown, and these detectors, as in Fig. 1, produce ythe differentialpotentials in I1 and I8 for automatic frequency control purposes. Thesedetectors 1 and 8 may have their grids biased to such values that thetubes operate on the curved portion of their characteristic curves tothereby obtain a more marked controlling result. 'I'his circuit alsodiffers from the circuit of Fig. 1 in that the anode of the modulatortube is coupled as shown by way of a variable capacity I5 to the anodeof the oscillator tube 5. Thus, in Fig. 2, a capacitive control of thefrequency of the high frequency oscillator 5 is obtained instead of aninductive control, as in Fig. k1. As the control voltage on the controlgrid of tube 6 is varied, the

yplate impedance of tube 6 is Varied so that the effective value ofcapacity I5 is varied. Since ca.- pacity I5 is, placed across the tuningcircuit of the high frequency oscillator, the variation of the effectivevalue of I5 varies the frequency of vthe high frequency oscillator. In`this manner, the tuning of the oscillator 5 and consequently of thereceiver circuit is varied until the differential detectors `1 and Barebalanced and the differential potentials in I1 and I8 oppose and areneutralized so that the control energy no longer flows in line L.`Battery 26 Yfurnishes bias for modulator 6. I6 is a radio frequencychoke.

In the modification of Fig. 2, the intermediate frequency signal energyis fed from the output of unit 3 by way cftransformer 23 to a rectifiertube I9, which is to supply energy for automatic volume controlpurposes. The filter circuit connected with the output electrodes of I9removes the intermediate frequency components from the rectifier output,and the rectifier output is set up across resistance 2|, connected asshown between lter circuit 20 and ground, the circuit being completed bygrounding the cathode of I9. TheV Voltage variations produced in 20 andappearing in 2l are fed by Way of an automatic volume control circuit(AVC) to the radio frequency amplifierV I, to determine therein thebiasing potential of a control grid, or some other electrode, of' a tubethereof to thereby determine the vamplification characteristic of theamplifier. The circuit AVC includes a time control device 22 whichdetermines the rapidity with which the variations inthe control circuitfollow the variations in thesignaling-pot-entials applied `to 23.

This circuit of. Fig. 2' further differs` from the circuit of Fig. 1 inthat thecontrol relay I0, which connects the line L to the output of1and 8 for materialA signal vpotentials and disconnectsV the sametherefrom for unsubstantial signal potentials, is supplied with energyfrom the resistor 2L The-relay is, however, actuated as in Fig. 1 by theplate current of tube 9. The control grid of tube 9 is connected by wayof a biasing source 28 to a point on 2l. When the signal is strong, thevoltage. across resistor 2l will be high and nega.-

tiveY so that tube 9 will be biased to a low valueV of plate current orcutoff. SpringsS will then close contacts Il and l2to apply thedifferential voltages from 'l and 8v to line L and thence to themodulator tube 6.` When the signal fades, the voltage across 2ldecreases towards zero,.so that tubeA 9V draws more plate current. AThespring-biased arms will then be pulled-toward the magnet of relay I9,thereby'opening the contacts H and l2 and removing the tuning controlenergy. Hence, by proper adjustment ofrelay l0 and the bias of thecontrol grid of 9, by adjusting battery 28 connected with the grid oftube. 9, the tuning control may be arranged to turn on at any desiredsignal level.

The tube 9 may be dispensed with, as shown in Fig. 3, by placing therelay I0 directly in the output circuit of rectifier I9 in series withresistance 2l. The relay contacts may then have to bechanged to theother side of the spring-biased arms in order to compensate for the 180phase reversal of tube 9. The same result can, of course, beaccomplished by reversing the direction in which the contact arms arebiased by springs S.

In the circuit of Fig. 4, condenser I5 may be replaced by an inductance30 in series with a blocking condenser 32. The plate resistance ofmodulator tube 6 is now in series with an nductance and the combinationof the inductance, blocking condenser and Variable plate resistance oftube 6 is placed across the oscillator tuned circuit, as shown. Thevariation of frequency of the oscillator 5 is in this case effected byvarying the effective inductance of the tuned circuit.

While I have indicated and described. several systems for carrying myinvention into effect, it will be apparent to one skilled in the artthat my invention is by no: means limited to the particularorganizations shown and described, but that many modifications may bemade Without departing from the scope of my invention, as set forth inthe appended claims.

What is claimed is:

l. In combination a receiver of the heterodyne type including a rstdetector and a local oscillator, a pair of differential detectors havinglike control electrodes coupled in phase and in phase opposition to theoutput of said rst detector, a transmission line and a modulator tubecoupling the output of said differential detectors to said localoscillator, contacts in said transmission line, and a relay tubecooperating with said contacts, said relay tube having its input.electrodes coupled to the output of said first detector.

2. In combination areceiver of the heterodyne type including a rstdetector and a local-oscillator and an intermediate frequency amplifier,a pair of differential detectors havingY likeY control electrodescoupled in-phase to said intermediate frequency ampli-iierand in phaseopposition to a filter connected to said intermediate frequencyamplifier, amodulator tube connected -,at.its input:to the output ofsaiddifferential decontacts in one of said connections, and. a relay tubecooperating with said contacts, said relay having its input electrodescoupled to the output of said intermediate frequency amplifier.

3. In combination a receiver of the heterodyne type including a firstdetector and a local oscillator, a pair of differential detectors havingtheir control electrodes coupled to the output of said first detector, atransmission Yline and a modulator tube coupling the output of saiddiiferential detectors to said local oscillator, a contact in saidtransmission line, a relay tube cooperating with said contact, saidrelayV tube having its input electrodes coupled to the output of saidrst detector, and automatic volume control means coupled between theoutput of said first detector and the input of said rst detector.

4. In combination a receiver including a first detector and a source oflocal oscillations, an in# termediate frequency amplifier coupled at itsinput to the output of said rst detector, a pair of differentialdetectors having like controlgrids connected in phase opposition to theoutput of said intermediate frequency amplifier, said detectors alsohaving like electrodes connected in like phase relation to the output ofsaid intermediate frequency ampliiier, a filter circuit in one of saidconnections, a frequency control circuit coupled to the output of saiddifferential detectors at its input, a modulator tube coupled to theoutput of said frequency control circuit,`a coupling between saidmodulator tube and said source of local oscillations, contacts in saidfrequency control circuit, a rectifier having its input electrodescoupled to the output of said intermediate frequency amplifier, ankelectromagnet' cooperating with the contacts in said frequency controlcircuit, and a circuit connecting said electromagnet to the output ofsaid rectifier.

5. In combination a receiver including a radio frequency amplifier, arst detector and a source of local oscillations, an intermediatefrequency amplifier coupled to the output of said rst detector, a pairof differential detectors having a pair of like control gridsconnectedin phase opposition to the output ofrsaid intermediatefrequency amplifier, said detectors also having other like electrodesconnected in like phase relation to the output of said intermediatefrequency amplifier, a filter circuit in one of said connections, afrequency control circuit coupled to the output of said differentialdetectors, a modulator tube coupled at its input to the output of saidfrequency control circuit, a coupling between said modulator tube andsaid source of local oscillations, contacts in said frequency controlcircuit, a rectifier having its input coupled to the output circuit ofsaid intermediate frequency amplifier, an automatic volume controlcircuit connecting the output of said rectifier to said radio frequencyamplifier, an electromagnet cooperating with the contacts 'in saidfrequency control circuit, and a circuit connecting said electromagnetto the output circuit of said rectifier.

6. In a receivena radio frequency amplifier,V a iirst detector coupledthereto, a source of local' oscillations coupled to said first detector,an intermediate frequency amplifier coupled to said first detector, alter coupled to said intermediate frequency amplifier, a pair ofdifferential detectors, circuits coupling the control electrodes of saiddifferential detectors in phase opposition to the output ofsaid=iilter,circuits connectingI tectors and at its output to said-localoscillator,l

the control electrodes inv said differential detectors in phase to theoutput of said intermediate frequency amplifier, a pair of resistorsconnected between the output electrodes of said differential detectors,a line including contacts connected to said pair of resistors, a timecontrol device in said line, a modulator tube connected at its input tosaid line, a reactive circuit connecting the output of said modulatortube to said source of local oscillations, an electromagnet cooperatingwith said contacts, a tube coupled at its input to the output of saidintermediate frequency amplifier and at its output to saidelectromagnet, and a demodulator coupled to the output of said filtercircuit and to the output of said intermediate frequency amplifier, oneof said last named couplings including a phase shifter.

'7. In a receiver, a radio frequency amplifier, a first detector coupledthereto, a. source of local oscillations coupled to said first detectorand an intermediate frequency amplifier coupled to said first detector,a filter tuned to the mean intermediate frequency coupled to saidintermediate frequency amplifier, a pair of differential detectors,circuits coupling like control electrodes of said differential detectorsin phase opposition to the output of said carrier filter, circuitsconnecting other like control electrodes in said differential detectorsin phase to the output of said intermediate frequency amplifier, a pairof resistors connected between the output electro-d-es of saiddifferential detectors, a line including contacts connected to saidresistors, a time control device in said line, a modulator tubeconnected at its input to said time control device, a reactive circuitconnecting the output of said modulator tube to said source of localoscillations, an electromagnet cooperating with said contacts, arectifier couple-d at its input to the output of said intermediatefrequency amplifier, a circuit coupling the output of said rectifier tosaid electromagnet, a filter circuit connected to the output of saidrectifier, and a volume control circuit coupling a point on said filtercircuit to said radio frequency amplifier.

8. In combination, a receiver including a first detector and a source oflocal oscillations, an intermediate frequency amplifier coupled at itsinput to the output of said first detector, a pair of differentialdetectors having like control electrodes connected. in phase oppositionto the output of said intermediate frequency amplifier, a circuitconnecting said like electrodes in phase to the output of saidintermediate frequency amplifier, a frequency control circuit coupled atits output to' the output of said differential detectors, a modulatortube coupled to the output of sai-d frequency control circuit and tosaid source of local oscillations, means for interrupting the couplingbetween said modulator tube and said frequency control circuit, and arelay coupling said last named means to the output of said intermediatefrequency amplifier.

9. In a radio receiver, a signal wave amplifier of controllable gain, ademodulator having an input coupled to said amplifier, said demodulatorhaving an output, tuning means for said demodulator, an automatic volumecontrol circuit coupling said demodulator to said amplifier to controlthe gain thereof, a tuning control circuit coupling the output of saiddemodulator to said tuning means to control the tuning of saiddemodulator, and a relay coupled to the output of said demodulator andto said control circuit to control the operativeness thereof.

10..In a superheterodyne receiver of the type including a detectorcircuit tuned to a desired signal frequency, a local oscillator circuithaving a tank circuit tuned to a frequency differing from the signalfrequency by an operating intermediate frequency, means coupling saiddetector and oscillator circuits, an auxiliary frequency determiningelement operatively associated with said tank circuit, means, responsiveto a frequency shift of the intermediate frequency energy from saidoperating value, for controlling said auxiliary element in a sense tomaintain said energy at said operating value, and additional means,responsive to a decrease in the signal carrier amplitude below a desiredlevel, for automatically rendering said controlling means ineffective.

11. In combination with a signal detector tuned to a desired signalfrequency, an oscillator circuit electrically associated with thedetector to produce beat energy in the detector output, main andauxiliary frequency determining elements operatively associated with theoscillator circuit, said main element tuning the latter to a frequencydiffering from the signal frequency by an assigned beat frequency, anautomatic frequency control circuit, responsive to a frequency shift ofthe beat energy from the assigned beat frequency, for varying themagnitude of said auxiliary element, and a signal controlled circuit,responsive to variations in signal carrier amplitude, for automaticallyregulating the effectiveness of said frequency control circuit.

12. In a combination with a signal detector tuned to a desired signalfrequency, an oscillator circuit electrically associated with thedetector to produce beat energy in the detector output, main andauxiliary frequency determining elements operatively associated with theoscillator circuit, said main el-ement tuning the latter to a frequencydiffering from the signal frequency by an assigned beat frequency, anautomatic frequency control circuit, responsive to a frequency shift ofthe beat energy from the assigned beat frequency, for varying themagnitude of said auxiliary element, and a signal-controlled circuit,responsive to variations in signal carrier amplitude, for automaticallyregulating the effectiveness of said frequency control circuit, saidsignal-controlled circuit comprising a signal rectifier, and means forapplying the rectified signals to said frequency control circuit.

13. In a radioreceiver of the superheterodyne type, a first detectornetwork, a local oscillator network, means for tuning the detector andoscillator networks to different station settings, an intermediatefrequency transmission network coupled to the first detector network,means for rectifying intermediate frequency energy appearing in saidintermediate frequency network and deriving therefrom a direct currentvoltage whose value is dependent upon the frequency difference betweenan operating intermediate frequency and the frequency of saidintermediate energy, frequency control means electrically connectedbetween said rectifying means and said oscillator network, andresponsive to said direct current voltage, for automatically adjustingthe frequency of the oscillator supplementally of said tuning means, andmeans, responsive to a decrease in the amplitude of the intermediateenergy below a desired level, for automatically rendering ineffectivesaid frequency control means.

14. In a radio receiver system of the superheterodyne type, a firstdetector network, a local oscillator network, means for tuning thedetector and oscillator networks to different station settings, anintermediate frequency transmission network coupled to the iirstdetector output circuit, means for rectifying signal energy appearing insaid intermediate frequency network and deriving therefrom auni-directional voltage whose magnitude is dependent upon the amplitudeof the intermediate carrier frequency, an automatic frequency controlcircuit having an input network coupled to said intermediatetransmission network and deriving from said intermediate energy a seconduni-directional voltage whose magnitude is dependent upon the differencein frequency between the operating intermediate frequency and the saidintermediate carrier frequency, additional means responsive to saidsecond uni-directional voltage for automatically adjusting the frequencyo-f the oscillator supplementally of said tuning means, and meansresponsive to said first uni-directional voltage for regulating theeffectiveness of said frequency control circuit.

15. In a radio receiver system of the superheterodyne type, a firstdetector network, a local oscillator network, means for tuning thedetector and oscillator networks to different station settings, anintermediate frequency transmission network coupled to the firstdetector output circuit, means for rectifying signal energy appearing insaid intermediate frequency network and deriving therefrom auni-directional voltage whose magnitude is dependent upon the amplitudeof the intermediate carrier frequency, an automatic frequency controlcircuit having an input .network coupled to said intermediatetransmission network and deriving from said intermediate energy a seconduni-directional voltage whose magnitude is dependent upon the differencein frequency between the operating intermediate frequency and the saidintermediate carrier frequency, additional means responsive to saidsecond unidirectional voltage for automatically adjusting the frequencyof the oscillator supplementally of said tuning means, means responsiveto said rst uni-directional voltage for regulating the effectiveness ofsaid frequency control circuit,

and additional means responsive to said lrst unidirectional voltage forcontrolling the transmission'efficiency of a portion of said receiverpreceding said intermediate transmission network in a sense to maintainthe carrieramplitude at the input of said frequency control 'circuitsubstantially uniform.

16. A method of signal reception in a receiver of the type including atleast a tunable selector network and a demodulator, which includes thesteps of collecting signal energy, impressing the collected energy onsaid tunable selector network to select signal energy of a desiredcarrier frequency, transmitting the selected signal energy to saiddemodulator, deriving a uni-directional voltage from the selected signalenergy which ls dependent in magnitude upon the frequency differencebetween the Selected carrier frequency and a predetermined carrierfrequency value, effecting an auxiliary tuning of said selector networkwith said voltage and in a sense to maintain the signal energytransmitted to the demodulator substantially at said predeterminedfrequency value, and utilizing the selected carrier energy to suppresssaid auxiliary tuning in response to a substantial decrease in carrieramplitude.

17. A method of signal reception in a receiver of the type including atleast a tunable selector network and a demodulator, which includes thesteps of collecting signal energy, impressing the collected energy onsaid tunable selector network to select signal energy of a desiredcarrier frequency, transmitting the selected signal energy to saiddemodulator, deriving a uni-directional voltage from the selected signalenergy which is dependent in magnitude upon the frequency differencebetween the selected carrier frequency and a predetermined carrierfrequency value, effecting an auxiliary tuning of said selector networkwith said voltage and in a sense to maintain the signal energytransmitted to the demodulator substantially at said predeterminedfrequency value, deriving a. second uni-directional voltage from theselected signal energy which is dependent in magnitude upon the selectedcarrier aniplitude, and suppressing the said auxiliary` tuning with saidsecond voltage.

MURRAY G. CROSBY.

